Effects of six codend meshes on the size selection of juvenile white croaker (Pennahia argentata) in demersal trawl fishery of the South China Sea

White croaker (Pennahia argentata) is a commercially important but overexploited species that is often caught in trawl fishery of the South China Sea (SCS). The codend size selectivity for this species in the local commercial trawl fishery is of concern when considering the established minimum landing size (MLS). This study investigated the size selectivity of white croaker for six different diamond-mesh codends with mesh size from 25 to 54 mm. We paid special attention to two codends, made with meshes of 25 and 40 mm in size, which are currently used according to the regulations established in the SCS. The results demonstrated that the legal codends do not perform satisfactorily in the fishing grounds where juvenile white croaker is relatively abundant. This is because at a length similar to the minimum landing size of the species (MLS = 15.0 cm), all white croaker were retained, and the estimated discard ratio was >97% in both cases of legal codends. Our study showed that by increasing the mesh size, the size selection of tested codends could be improved for white croaker, and the retention rates for juvenile fish would decrease. However, none codend was proved efficient to release undersized white croaker suggesting that other gear design changes may be necessary.

Application of Combined Analyses of Stable Isotopes and Stomach Contents for Understanding Ontogenetic Niche Shifts in Silver Croaker (Pennahia argentata)

Stable isotope analysis (SIA) and stomach content analysis (SCA) were conducted to understand ontogenetic niche shifts in silver croaker Pennahia argentata inhabiting the southern coastal waters of the Korean peninsula. Sampled P. argentata were classified into three groups based on their total length (TL; 60–80 mm TL, 80–120 mm TL, and 120–210 mm TL). Carbon isotope (δ13C) ratios were distinguishable, whereas nitrogen isotope (δ15N) ratios were not significantly different among size classes, and Standard Ellipse Area (SEA), estimated by δ13C and δ15N, was expanded with increasing TL from 0.2 ‰2 (60–80 mm TL) to 2.0 ‰2 (120–210 mm TL). SCA results showed variable contribution of dietary items to each size class. In particular, higher dietary contribution of Polychaeta to P. argentata of 80–120 mm TL than 120–210 mm TL mirrored variation in δ13C values of P. argentata in those size classes. Based on the combined analyses involving SIA and SCA, we concluded that P. argentata underwent ontogenetic niche shifts, particularly dietary shifts, with growth stages. Ontogenetic niche shifting is a representative survival strategy in fish, and, therefore, represents essential information for managing fisheries. The present study demonstrated applicability of combined SIA and SCA analyses, not only for dietary resource tracing, but also for ecological niche studies.

Juvenile fish assemblages in the Jinju Bay region, Korea

Assemblages of juvenile fish and associated abiotic parameters were investigated inside and outside Jinju Bay in southern Korea, on a monthly basis from December 2014 to November 2015. Fluctuations in water temperature and salinity were larger inside than outside the bay. In total, 534,657 individuals per square kilometre from 81 fish species and 47 families were collected during the study period. The most dominant species was Nuchequula nuchalis both inside (25.6%) and outside (26.9%) the bay. The next dominant species were Thryssa kammalensis (17.9%) and Zoarces gillii (16.0%) inside the bay and Liparis tanakae (16.9%) and T. kammalensis (9.0%) outside the bay. Forty species (33% of total number of individuals) of young fish were recorded inside the bay and 47 species (52%) outside the bay. Therefore, it appears that a diversity of fish use nursery grounds inside and outside Jinju Bay. In particular, the following six species appeared: Z. gillii, Pleuronichthys cornutus, L. tanakae, Hemitripterus villosus, Pennahia argentata, and Xenocephalus elongates. Due to assemblage differences for fishes within Jinju Bay and outside the bay, management of both areas is required to maintain current diversity of species in the region.

Estimation of Direction of Arrival of Fish Calls in a Chorus Using Stereo Hydrophones

Some fish species produce sounds consisting of periodic pulses that are associated with distinct behaviors (e.g., courtship). Thus, the ability to estimate the direction of arrival from recorded sounds could provide insight into fish ecology and behavior. A stereo recording system was used to monitor underwater fish call sounds. The recorded signals were subjected to automatic processing to detect sounds that had the characteristics of fish calls. The direction of arrival for each detected sound was then estimated using the arrival time difference between the two hydrophones. Simulations confirmed that the recording system could accurately estimate the direction of arrival of fish call sounds. Furthermore, a blind source separation method was used to separate detected sounds originating from multiple individuals or groups of fish. Using this method, the direction of arrival for calls from multiple sources was estimated. The method described in this paper was successfully applied to monitor and isolate sounds from white croaker (Pennahia argentata) in the ocean.